1. Field of the Invention
This application relates to whistles, and in particular, to a whistle that produces audible sounds when a very low pressure differential exists across the whistle.
2. Description of the Prior Art
Various techniques have been used in the detection of low pressure differentials in air flows. Such devices include mechanical transducers having moving parts that respond to and thus indicate the level of the sensed pressured differential. One such type of mechanical pressure measurement is by means of a Bourdon tube. The most simple form of Bourdon tube is a hollow flat double ribbon that is bent into an arc which has a lever mechanism operated pressure indicator that is connected to a closed end of the tube. A pressure change at an inlet or open end of the tube causes the tube to straighten and thus, through the action of the lever mechanism causes the pressure indicator to move. A characteristic, however, of such devices is that they are subject to malfunction due to friction between the moving parts and deterioration because of age.
A second type of pressure indicator is a diaphragm or bellows gauge. The pressure is sensed by the responsive movement thereto of flexible metallic diaphragms, that are usually formed into flat, disk-like capsules and often having several such capsules stacked together.
As in the case of the Bourdon gauge, the pressure indicator actuating force that is due to the pressure in the gauge, is usually balanced by the elasticity of the diaphragm and also by the pressure existing at the exterior surface of the pressure sensing element.
The described devices, although capable of pressure measurement, however, suffer from the inability to detect pressure differentials in air flows, because the devices are sealed and thus do not permit air to flow therethrough. For the purposes of this application, air flows are intended to be synonymous with and include any gaseous flow.
Pressure differentials in air flows arise typically across filters as would be found in furnaces, air conditioning systems and automobile carburetors. When the particulate matter trapped in such filters restricts the air flow, a pressure differential across the filter results. Increasing particulate concentrations within the filter eventually prevent proper air flow through the filter giving rise to increased pressure differentials and degraded filter performance.
Additionally, the devices in the prior art suffer from an inability to detect small pressure differentials especially in the range of 0.005 to 0.02 psi. Generally, such devices respond to pressures greater than 0.5 psi.
Further, the devices described require either visual observance of the pressure indicators or require complex mechanical or electrical techniques for pressure indication.